1. Field of the Invention
The present invention relates to a connecting lead wire for a solar battery, a method for fabricating the same, and a solar battery using the connecting lead wire, in more particularly, to a connecting lead wire to be connected to a predetermined contact region of a Si cell in a solar battery, a method for fabricating the same, and a solar battery using the connecting lead wire.
2. Related Art
In a solar battery that is the mainstream in recent days, a Si cell comprising a polycrystal or single crystal Si crystal wafer is used.
FIG. 2 is a perspective view showing a state where a connecting lead wire for a solar battery is connected to a Si cell.
As shown in FIG. 2, a solder plated rectangular wire 2 (connecting lead wire for a solar battery) is joined with a predetermined region of a Si cell 1 by a solder, through which an electric power generated in the Si cell 1 is transmitted.
FIG. 3 is a lateral cross sectional view of a conventional and typical connecting lead wire for a solar battery.
As for the solder plated rectangular wire 2, there is a rectangular conductor in which a pure Cu such as tough pitch Cu or oxygen-free Cu is used as a conductor 3 and a solder plating 4 comprising Sn—Pb eutectic solder is used as shown in FIG. 3. Japanese Patent Laid-Open No. 11-21660 discloses this type of the rectangular conductor wire for a solar a solar battery.
In addition, it has been studied to change from a Pb-based solder to a Pb-free solder with considering the affect on the environment. Japanese Patent Laid-Open No. 2002-263880 discloses a typical Pb-free solder and a connecting lead wire using the same. Similarly, the conventional solar batteries and the connecting lead wires using a Pb-free solder are disclosed by Japanese Patent Laid-Open Nos. 2005-243972, 2006-49666, 2006-140039, 2006-54355, and 2005-243935.
Among members composing the solar battery, the Si crystal wafer holds most of a material cost of the solar battery. Therefore, it has been studied to reduce a plate thickness of the Si crystal wafer. However, when the plate thickness of the Si crystal wafer is reduced, there are disadvantages in that the Si crystal wafer is warped or damaged due to the temperature change in a heating process for bonding the connecting lead wire to the Si cell or in the use of the solar battery.
FIGS. 4A and 4B are explanatory diagrams showing connecting states of the Si cell and the connecting lead wire for a solar battery, in which FIG. 4A shows a state before solder bonding, and FIG. 4B shows a state where a warping occurs after solder bonding. For example, as shown in FIG. 4A, warping does not occur in the Si cell 1 and solder plated rectangular wire 2 and both of them are straight, before the solder bonding. However, as shown in FIG. 4B, the warping occurs in the Si cell 1 after the solder bonding.
So as to solve this problem, a demand for a material with a low thermal expansion as a material of the connecting lead wire is elevated.
FIG. 5 is a lateral cross-sectional view of a conventional connecting lead wire for a solar battery using CIC (Cu-invar-Cu).
When a cladding material CIC, in which Cu 3′ and Fe-36 mass % Ni (generally called as “invar” (registered trademark)) 5 are laminated as cladding layers in order of Cu-invar-Cu as shown in FIG. 5, is used as the rectangular conductor, since the invar has a low thermal expansion as shown in TABLE 1 describing material properties of Cu-invar-Cu, Cu, invar, and Si, it is possible to realize a thermal expansion matching with Si. However, an electric conductivity of invar is lower than that of Cu (namely, a volume resistivity of invar is higher than that of Cu), so that a generating efficiency of the solar battery is deteriorated.
TABLE 1Cladding materialCu-invar-CuPlate thicknessmaterialratio 2:1:2CuinvarSiThermal expansion13.117.01.83.5Coefficient (×10−6/° C.)Young's Modulus (MPa)125120145170Volume resistivity (μΩ · mm)21.016.98102.3 ×109
As a technique for realizing both of a high electric conductivity and a warping suppression, it may be possible to reduce a “cell warping force” generated by a thermal shrinkage of the conductor after the solder bonding, by reducing 0.2% proof stress value of the conductor with a high electric conductivity. According to this technique, the thermal expansion matching with Si can be realized. However, the 0.2% proof stress values of the conductor coated with the solder 4 is increased compared with that of the conductor before solder plating, so that it is difficult to realize a further reduction in the thickness of the cell.